The present invention relates to a phase control device, and particularly, to a phase control device suitable to use in regulating an illumination of a fluorescent lamp.
An example of the conventional phase control device for illumination regulation of a fluorescent lamp is shown in FIG. 1. In FIG. 1, an anode of a thyristor 1 is connected to a gate of a triac 2 and a gate of the thyristor 1 is connected to a base of a unijunction transistor 3. An output pulse of the transistor 3 triggers the thyristor 1 and, during a conduction thereof, an a.c. voltage applied to a load 4 is phase-controlled by a gate current of the triac 2. A resistor 5 is selected such that a current flowing therethrough does not exceed a forward rate current of the thyristor 1, does not affect a gate characteristics of the triac 2 adversely and does not destroy a Zener diode 6. A variable resistor 7 is used for determining the degree of the phase-control. That is, a value of the variable resistor 7 is manually set to determine the phase angle after an ignition of the thyristor 1, to thereby regulate an illumination of the fluorescent lamp, i.e., the load 4 suitably.
This conventional phase control device, however, is relatively complicated in construction and is relatively not stable because the phase control is conducted simultaneously with a turning-on of the lamp 4 which requires a certain time enough to warm up a hot cathode thereof.